This invention relates to the catalytic cracking of hydrocarbons in the presence of a fluidized phase catalyst. More particularly its objectives are a method and device that make it possible to introduce a homogenous flow of catalyst grains in the reaction section, at the injection level of the charge to be cracked.
As known, the oil industry uses heavy hydrocarbon charge conversion methods in which hydrocarbon molecules with a high molecular weight and high boiling point are split into smaller molecules that can boil in lower temperature ranges that befit the sought use.
In this field, the method most used is currently the method called fluid catalytic cracking (or FCC method). In this type of method, the hydrocarbon charge, pulverized into fine droplets, is put in contact with high temperature cracking catalyst grains that circulate in the reactor in the form of a fluidized bed, meaning in a more or less heavy medium within a gaseous fluid that ensures or helps their transport. On contact with the hot catalyst, the charge vaporizes, this is followed by the cracking of the hydrocarbon molecules on the active sites of the catalyst. Once the desired molecular weight range has been reached in this manner, accompanied by a depression of the corresponding boiling points, the effluents of the reaction are separated from the catalyst grains. These catalyst grains, deactivated as a result of the coke that has deposited on their surface, are then stripped in order to recuperate the hydrocarbons that have been swept away, then are regenerated by combustion of the coke, and lastly, are put back in contact with the charge to be cracked.
The reactors used are generally vertical reactors of the tubular type, in which the catalyst moves following a flow that is mostly upward (the reactor is then called xe2x80x9criserxe2x80x9d) or following a flow that is mostly downward (the reactor is then called xe2x80x9cdropperxe2x80x9d or xe2x80x9cdownerxe2x80x9d).
We know that one of the key factors of the catalytic cracking process lies in the quality of the mixture of the charge that is injected in liquid state in the form of fine droplets, with the flow of hot catalyst grains resulting from the regeneration. Indeed, it is essential to ensure that the hydrocarbons are quickly, closely and evenly put in contact with the catalyst flow, as this determines the efficiency of the thermal transfer from the hot catalyst grains to the droplets of the charge. The speed and evenness of the vaporization of the charge depend on the efficiency of this transfer, and therefore, so does the quality of the conversion of the charge since the catalytic cracking reaction takes place in the gaseous state.
However, the studies completed in this field by the applicant have revealed that the yields obtained with the highest performing cracking units remain below what was predicted by the theoretical studies and that this difference is due among other things to the fact that the droplets of charge were not put in contact with the catalyst particles in an adequate fashion. We assumed that it was in part due to an inhomogeneity of the density of the fluidized bed of the catalyst that arrives in the injection area of the charge, or in other words to a signification segregation within the mixture consisting of the catalyst grains and the gaseous fluid that ensures their transportation.
In particular, we have illustrated two main factors of segregation:
On the one hand, in conventional devices, the circulation pattern of the catalyst grains often lacks stability. In particular, the catalyst grains emanating from the regenerator tend to show up xe2x80x9cin bunchesxe2x80x9d and a phenomenon called pulsation phenomenon is then noticed: the feeding of catalyst grains into the reactor is not continuous, and the density of the catalyst flow arriving in the cracking zone may then fluctuate considerably in time around an average value. This pulsating pattern shows a fluctuation in time of the actual C/O ratio of the quantity of catalyst C introduced in the reaction zone to quantity O of the injected charge to be cracked.
On the other hand, in particular for units equipped with a upward flow reactor (riser), in the slanted conduit that ensures the transfer of the regenerator""s catalyst grains toward the reactor, these grains tend to gather on the bottom, while the conveyor vapor creates xe2x80x9cpocketsxe2x80x9d in the upper part of this transfer conduit. The elbow that is present at the point of connection between this slanted conduit and the reactor only accentuates the segregation. As the catalyst""s fluidization device that is present at the entry of the reactor does not allow for the re-balancing of the distribution of catalyst grains on the section of the reactor, we notice, for one same section, an inhomogeneity of the density of the catalyst. The results in an inhomogeneity of the actual C/O ratio and therefore of the temperature profile for one same section of the reactor.
At the injection area of the charge, these spatial and temporal variations of the actual C/O ratio have proved to be particularly problematic, since they lead to an inhomogeneity of the vaporization and the cracking of the injected charge. In areas where the density of the catalyst is too high, the charge runs the risk of overcracking, which generates dry gases and coke at the expense of the sought intermediate hydrocarbons. In return, in area where the density of the catalyst is insufficient, the charge is only partially vaporized, which leads to an increased deposit of hydrocarbons on the surface of the catalyst, by collision of the catalyst grains with the non vaporized droplets of the charge, from which results a greater coking of the catalyst. In other respects, the deficit in catalytic sites favors the thermal cracking reactions, which are not very selective, at the expense of the catalytic cracking reactions.
In the end, all these phenomena translate into a significant penalty in terms of conversion yields and selectivity, and lead to a significant coking of the separation and stripping chamber and inside the reactor
In order to remedy the problems described above, the applicant has already proposed a certain number of solutions.
In patent EP-0 326 478, we proposed a new form for the connection that ties the regenerator to the reactor of a catalytic cracking unit that operates in the upward mode. In particular, this connection consists of tubing connected by incurved elbows that dictate neither an upturning point on the particles"" path, nor sudden modifications of the tubing diameter. Injections of a make up carrier gas are also planned in order to accelerate the catalyst particles in a controlled manner at the level of the reactor""s connection elbow. By using this process we can connect the catalyst transfer line and the upward reactor following a curvilinear profile, which makes it possible to limit the dehomogenization of the catalyst and of the fluid that ensures its transport, but it does not however, make it possible to completely eliminate the inevitable segregation that takes place in a solid/gas biphasic mixture, nor the pulsating pattern of the circulation of the catalyst. Furthermore, it corresponds to an optimization of the configuration of a unit that contains a reactor that operates in an upward mode, and therefore in no way affects the unit where the reactor operates in a downward mode.
In patent EP-0 191 695, the applicant has described an advantageous fluidization system in two steps at the base of an upward reactor. The proposed solution consists in slowly injecting a first fluid into the reactor, below the level of introduction of the catalyst grains emanating from the regeneration area, in order to maintain a dense and homogenous catalyst fluidized bed at the bottom of the xe2x80x9criserxe2x80x9d, and in simultaneously injecting a second fluid below the upper part of the dense catalyst bed, in order to obtain a more diluted and homogenous fluidized phase with a constant flow of catalyst grains, upstream from the injection area of the charge. Such a procedure, although efficient, does nevertheless present considerable inconveniences. It consumes large amounts of fluidization vapor and often results in an excessive dilution of the suspension of catalyst grains, which can be harmful to the vaporization speed of the charge to be cracked and therefore to the conversion of the latter. Furthermore, once again, the procedure was specifically intended to answer the fluidization problems encountered in an upward flow reactor (xe2x80x9criserxe2x80x9d) and can in no way be transposed to cases using downward flow reactors (xe2x80x9cdownersxe2x80x9d).
This invention proposes to remedy these inconveniences using a device that makes it possible to introduce a homogenous and stable flow of catalyst particles in the injection area of the charge to be cracked, whether this reactor is an upward or downward reactor.
Consequently, the object of the invention is a device for introducing fluid state catalyst particles in a catalytic cracking reactor equipped, in its upstream part, with at least one means of feeding catalyst particles that are at least partially regenerated, and at least one means of injecting the charge to be cracked, where this device is characterized by the fact that said upstream part of the reactor contains, between the feeding area of the catalyst flow and the injection area of the charge to be cracked, at least one solid and attached packing element that extends over all or part of the reactor""s cross section and that consists of a network of cells through which pass the catalyst particles, where said network ensures at least one step in the dividing and recombining of the flow of catalyst particles, so as to redistribute the latter in a homogenous manner on the cross section of the reactor.
In this description, by network of cells, we mean a set of at least two cells and, preferably, of a large number of cells, disposed side by side and that may or may not be identical in size.
Furthermore, by reactor we mean the tubular type vertical chamber in which the charge to be cracked is put in contact with a flow of catalyst particles that move following a flow that is essentially upward (xe2x80x9criserxe2x80x9d type reactor) or following a flow that is essentially downward (xe2x80x9cdropperxe2x80x9d or xe2x80x9cdownerxe2x80x9d type reactor).
In the invention""s device, the reactor contains one or several solid and attached packing elements, meaning they do not contain any mobile parts. Preferably, each packing element extends over the entire cross section of the reactor.
Such a packing element can advantageously consist in a grid. The element then ensures a division followed by a recombination of the flow of gas and particles that passes through it.
It can also consist of a system of intersecting and stacked bars possibly soldered to each other. The bars that are used may have any type of section; advantageously they will have a rounded section, in order not to present any sharp angles that would result in abrasions caused by the flow of catalyst particles.
The preferred packing elements are elements of the static mixer type. Indeed, such an element by itself and over a very short distance ensures an entire series of successive divisions/recombinations of the flow of gas and particles that passes through it.
A first advantage of the invention""s device as compared to that of the prior art is that it makes it possible to avoid having to introduce too much fluidization vapor, as in patent EP-0 191 695, which not only reduces the operational constraints of the unit but also proves to be very beneficial in terms of operational costs of this unit. Furthermore, it also makes it possible to avoid the risk of an excessive dilution of the flow of catalytic particles, which can be harmful to the efficiency of the vaporization of the charge. Last but not least, it ensures, in a minimum of space, an optimal stirring of the catalyst""s fluidized bed.
A second advantage of said device concerns its universal character: it can indeed be applied to any type of reactor, whether it be a xe2x80x9criserxe2x80x9d or xe2x80x9cdownerxe2x80x9d type reactor, without the need for any substantial modifications to the latter since all that must be done is to insert said packing. Easy to implement and fairly low in cost, the invention""s device is therefore extremely advantageous in the frame of the modernization of existing units.
Another objective of the invention concerns the procedure associated with the above-mentioned device. In this procedure of catalytic cracking of a hydrocarbonaceous charge, we feed a tubular type reactor, whose flow is an essentially upward or downward, with catalyst particles that are at least partially regenerated, in the form of a fluidized bed into which we then inject the charge to be cracked. This procedure is characterized by the fact that, between the feeding of the catalyst flow and the injection of the charge to be cracked, we have provided for at least one step consisting in one or several simultaneous divisions of the flow of catalyst grains followed by a recombination of said flow, so as to redistribute the latter in a homogenous manner over the cross section of the reactor.
The method and device as set forth in the invention make it possible to reach the afore-mentioned objectives. Indeed, they make it possible to obtain one, and preferably several successive stages of division and recombination of the flow of catalyst grains, which results in radially mixing and homogenizing the catalyst flow that penetrates the injection area of the charge over the entire section of the reactor.
Furthermore, and surprisingly so, the invention has proved to have a stabilizing effect on the feeding of the reactor in catalyst particles. Indeed, we note a disappearance of the rapid pulses of the catalyst flow that penetrates the injection area of the charge. This continuity of the feeding in catalyst is fundamental, more so when, as is often the case, we seek to reduce the duration of contact between the charge and the catalyst.
The homogenization and stabilization of the feeding of the reactor with catalyst flow result in a better homogeneity of the actual C/O ratio in the injection area of the charge, which leads to an improvement of the quality of thermal transfers between the droplets of charge and a more homogenous and continuous catalyst flow. The velocity and uniformity of the vaporization of the charge are therefore improved and thus we note a reduction in the local phenomena of overcracking and insufficient vaporization. So, we not only increase the rate of conversion of the charge but also the selectivity of this conversion: fewer lighter products (such as dry gases that are not very amenable to beneficiation) and less coke are created.
This reduction in the quantity of coke that is created when the charge is put in contact with the catalyst is all the more noticeable because it makes it possible, on the one hand, to have a better control of the thermal balance of the unit, and on the other hand, to avoid a premature deactivation of the catalyst through neutralization of the catalytic sites. Furthermore, it makes the catalyst regeneration step easier by reducing the risk of appearance of hot spots that could harm the catalyst, and by reducing the risks of reaching temperature levels in the regenerator(s) that are particularly high and incompatible with the metallurgy of the unit.
Another advantage of the invention is tied to the improvement of the vaporization process of the charge, which makes possible the conversion of very heavy cuts in the catalytic cracking unit, such as for example residue. Indeed, these charges boil at particularly high temperatures and are known for being hard to vaporize under the conditions of the cracking reaction. By proposing a better contact between the droplets of the charge and the grains of the catalyst, the invention allows for a better vaporization of said charge, which makes it possible to convert charges that contain much higher quantities of heavy products. The flexibility and profitability of the catalytic cracking unit are thus increased.
Other advantages of the method and device as set forth in the invention will become apparent in the more detailed description of a few preferred methods of realization.
Preferably, the device as set forth in the invention contains one or several packing elements, so as that a large number of successive divisions and recombinations of the flow of catalyst grains can be attained upstream from the injection area of the charge.
Preferably, the divisions of the flow of catalyst grains must be as fine as possible, while taking into account the requirements tied to the circulation of particles, the constraints of the process and the space available to set up the packing element(s) that ensure said divisions.
Each packing element is positioned on the reactor""s cross section, perpendicular to the axis of the latter, and consists of a network of cells that make it possible to obtain, one if not several successive stages of division/recombination of the flow of catalyst particles that passes through said cells. The section of these cells is chosen based on the size and the speed of passage of the catalyst particles, in order to avoid an obstruction inside the packing element, which could hinder the circulation of the catalyst flow.
According to one preferred mode of realization, the packing element consists of a network of cells where each cell guides in a considerably radial manner in relation to the axis of the reactor, the direction of the flow of gas and particles that passes through it. These cells may take on many forms.
In particular, the static mixer type packing elements, usually used in other fields, can be adapted and make particularly efficient elements.
Advantageously, we will use a packing element that consists of a network of cells set up so as to induce deviations in two different types of directions: more or less 50% of the flow of gas and catalyst is diverted in a first direction and more or less 50% of the flow of gas and catalyst is diverted in a second direction that forms a 10 to 90 degree angle with the first direction.
This deviation and this radial orientation are induced by the geometry of said cells, disposed to form a network. Such a network can advantageously consist of an assembly of corrugated sheets, cut crosswise in relation to their planes. These corrugated sheets are molded or soldered in such a way that the ridge of the corrugation of each sheet creates an angle of more or less 45 to 135 degrees with the ridge of the corrugation of the sheet adjacent to it. According to one preferred mode of the invention, this angle is a 90 degree angle, thus defining a network of right angle intersecting canals. By ridge of the corrugation, we mean the right section consisting of the top of a corrugation of the sheet.
This disposition in intersecting canals advantageously makes it possible to divert the particles at each intersection between the tops of the corrugations of a first sheet and the tops of an adjacent sheet, thus improving the divisions and recombinations of the particles and the fluid surrounding them. The packing element that is created in this manner has the advantage of creating by itself an entire series of successive divisions/recombinations, optimized so as to result in a good mixture and a homogenization of the catalyst particles and the gaseous fluid within the fluidized bed.
Static mixers of the type marketed by Sulzer-SMV or Kenics can be adapted in order to create such a packing.
Other types of static mixers can also make excellent packing elements, such as the static mixers consisting of one or several helix fragments. Advantageously, said static mixer consists of several helix fragments side by side and staggered in rotation.
According to a mode of realization particularly advantageous, the catalytic cracking reactor contains at least two, and preferably from two to four packing elements, which can be identical or not.
These packing elements may be side by side or, on the contrary, be spaced at a distance based on their nature and on the geometric constraint of the procedure.
If at least two identical packing elements are present, they will preferably be staggered in translation and/or in rotation, so that the networks of each are not literally on top of each other.
For example, if said elements consist of assembled corrugated sheets, the plane of the corrugated sheets of a first element will preferably be oriented so as to form an angle of more or less 45 to 90 degrees with the plane of the corrugated sheets of the closest second element of the same type.
The packing elements involved in the device as set forth in the invention must remain intact in the severe conditions of implementation of the fluid phase catalytic cracking process. In particular, they consist of one or several materials that are able to resist heat and erosion, such as high temperature steel and ceramic.
Advantageously, the packing elements are positioned, in the vertical part of the reactor located upstream from the injection of the charge, at a level such that the flow of catalyst grains that passes through them is in the form of a fluidized phase, whose density can density can be adjusted to a value ranging between 200 and 800 kg/m3 through injection of fluidization vapor.
These packing elements are positioned upstream from the injection area of the charge: the distance that separates the downstream packing element from the upstream injectors preferably ranges between 0.3 and 3 times the average diameter of the reactor.
A particularly advantageous alternative of the process as set forth in the invention consists in later rehomogenizing the flow of catalyst particles immediately downstream from the injection of the charge to be cracked, by recentering the catalyst particles in the direction of the axis of the reactor. This allows for the correction of the segregation induced by the injection of the charge and the very fast vaporization of the latter, which tend to project and concentrate the catalyst on the walls of the reactor. Thus, we now have a real mixing chamber, that contains means of homogenization of the fluidized bed of the catalyst both upstream and downstream from the injection of the charge, which can make it possible to optimize even more completely, the efficiency of the contact between the catalyst and the hydrocarbons to be cracked.
In order to homogenize the reaction mixture downstream from the injection of the charge, we can use any means known that makes it possible to deviate the path of the catalyst particles and preferably push them back in the direction of the central axis of the reactor.
This can advantageously be done by using one or several profiled circular obstacles, positioned over the entire periphery of the internal wall of the reactor, that ensure there will be one or several local narrowings of the section of the latter. These obstacles can advantageously be annular or helicoidal, with rounded section, for example hyperbolic, in a semi-circle or even in a semi-ellipse. Thus, the absence of sharp angles avoids any risk or erosion of said obstacles by the flow of catalyst particles.
We can also use one or several packing elements such as those used in the homogenization of the fluidized bed of the catalyst upstream from the injection of the charge and that are described previously, such as grids, intersecting bars, and elements of the static mixer type.
We can also have recourse to a recentering gaseous fluid injection device, positioned on or in the internal wall of the reactor and as described in patent EP-0 485 259, in the name of the applicant.
So, in order to strengthen the efficiency of the process as set forth in the invention, we can have recourse to one or several successive means of re-homogenization of the reaction mixture, positioned immediately downstream from the injection of the charge: preferably, these means are located at a distance ranging between 0.5 and 2 times the average diameter of the reactor from the injectors the most downstream.
In this description, the mode of injection used is not specified and can be any mode of injection known in the industry. In particular, the injection of the charge to be cracked can be perfectly executed with the current and/or against the current in the overall direction of the circulation of the catalyst in the cracking reactor (see for example patent EP-0 209 442, in the name of the applicant).
Likewise, the type of catalyst used is not mentioned nor is the means for circulating the latter in the form of a fluidized bed more or less diluted by dilution gaseous fluids which is data well known to a person experienced in the field.
Furthermore, the experienced person will definitely be able to adapt the device and the method that are the objects of this invention to procedures related to catalytic cracking, such as for example, procedures where thermal cracking type reactions are carried out by putting a hydrocarbon cut to be converted in contact with a fluidized phase of heat carrying particles in a tubular type reactor.
Various forms of implementation of the invention will be described in more detail later, while referring to the attached drawings. These are only intended for purposes of illustrating the invention and are therefore not intended as limitative in any way as the device and method that are the objects of this invention may be implemented according to a great many alternatives.